The invention concerns the biometric identification of a user of a system. It is adapted to an identity check on a portable object of the chip card type, e.g. so-called smart cards.
One method normally used for identifying a user is based on a secret identification code, also referred to as a PIN (Personal Identification Number). In a system using chip cards, a user enters his PIN code on a transaction terminal, which then transfers the entered PIN code to the chip card, which checks the PIN code by comparison with a reference PIN code. The security of such a system is guaranteed by the fact that the reference PIN code is stored within the chip card, which is inherently protected, and never leaves it during the identity check process, since the final check is made by the card. The drawbacks of such a system are the fact that the owner of the card must memorize a secret and the fact that another user can commit fraud by stealing this secret.
Biometrics consists of acquiring, measuring and recognizing physical characteristics of a user. It makes it possible to directly identify a user whilst the PIN code method allows indirect identification by the fact of checking that the user knows a secret. Amongst the known techniques in biometrics, there are the methods of recognizing voice characteristics, characteristics peculiar to the shape of the face or to the iris of the eye or, in the most frequent case, fingerprint characteristics.
All the existing biometric identity check systems are broken down into three phases:                the first phase is a phase of capturing biometric data from a sensor. The obtained biometric data are usually images, for example in the case of fingerprints, iris or face shape. However, it can also be a case of sound sequences in the case of voice recognition.        The second phase is an analysis or extraction phase for extracting a biometric signature from biometric data captured during the first phase, this signature being composed of a more restricted set of particular biometric data. This second phase is extremely complex and requires high calculation power.        The third phase consists of comparing the biometric signature obtained during the second phase with a reference signature defined previously during a procedure called enrolling.        
Portable electronic objects of the chip card type are provided with microprocessors whose calculation power is still limited. This is why a biometric system of the prior art using chip cards functions as follows:                in the enrolling phase, a reference biometric signature of the user of the card is stored in a secure memory of the chip card.        in the identity check phase, a terminal captures the biometric data of the user and then extracts a biometric signature. Two solutions then exist:        either the reference biometric signature is transferred from the card to the terminal for a check on the identity, by comparing the two signatures, on the terminal: this method has the drawback that the reference signature may be intercepted, which is a security failing. A conventional identification by PIN code does not have this drawback;        or the extracted biometric signature is transferred to the card for a check on the card. The card having only a low calculation capacity, the check is a complex operation, which requires a long processing time. This represents a drawback compared with the conventional use of a PIN code.        